How would you define a parasite?

Defining a parasite involves understanding its ecological role and relationship with a host. A parasite is characterized as an organism that lives in or on another organism, known as the host, deriving nutrients at the host's expense. This relationship typically harms the host, as the parasite benefits from the nutrients, shelter, or other resources provided by the host organism.

In contrast, the other options represent different types of ecological relationships. For example, a plant that grows on another might be a form of epiphyte or maybe a hemiparasite, which does not always harm the host to the same extent as a true parasite. Decomposers, on the other hand, break down dead organic matter and recycle nutrients back into the ecosystem, without living off a host in the parasitic sense. Lastly, mutualistic organisms engage in a symbiotic relationship where both the organism and its partner benefit, which is fundamentally different from the exploitative nature of parasitism. Thus, the definition that specifies an organism living off a host captures the essence of what a parasite is and clarifies its role in ecological interactions.